When the rock seems unbreakable, stresses are multiplied: The cutters must be stronger, the TBM more durable, and the operation optimised to keep equipment running smoothly. What qualifies as extremely hard rock? “Anything above 40MPa, we recommend disc cutters rather than soft ground tooling,” explains Robbins cutter product manager Sindre Log. “Anything over 100 MPa is hard rock. And when we get into the realm of 180 to 200MPa, we have extremely hard rock.” This type of rock, says Log, requires a different kind of rock breaking. “We’re getting closer to the limit of the cutters, and so you will see more destructive wear. That is where you really see the difference in quality of tool steel and cutter design.”
Properly designed equipment will give contractors an advantage in extreme rock conditions, but TBM operation and maintenance are equally important: “In very hard rock conditions, a systematic approach to cutter change and cutterhead maintenance is imperative,” said Andy Birch, Robbins Field Service Supervisor. “A skilled and diligent cutter change team will ensure acceptable excavation rates can be maintained, minimise cutter damage, excessive wear and damage to the cutterhead, and help minimise TBM downtime.”
Given the clear risks of excavating massive, hard rock, how can tunnellers set themselves up for the best possible chance for success? The combination of knowledgeable personnel, properly designed equipment, and proper TBM operation and maintenance are making excavation of hard rock – even extremely hard rock strengths of 300 MPa UCS or more – possible.
MAKING THE CUT
Cutters are a significant factor for efficient excavation when rock is extremely hard. To that end, Robbins has developed its Extra Heavy Duty (XHD) rings for projects where Heavy Duty (HD) rings are close to their design limit in terms of the thrust force required to break the rock. The XHD rings resist chipping, can occur in very hard rock conditions. Enhanced heat treatment gives the discs increased hardness and strength without the normally-associated reduction in fracture toughness.
Standard solutions in extremely hard rock formations include increasing the tip width of the cutter ring, or increasing the flank angle of the ring tip to reduce the stress induced in the ring. However, these measures are a compromise on the performance as the stresses induced in the rock are also reduced and the cutters cut the rock less efficiently. “Factors that often are not taken into account when considering cutters are the ring wear and flank angle.
As an example, Robbins was recently involved in a project where another manufacturer’s cutter was tested. Even though the cutter tip width was ¾” (~20mm) for both cutter rings, the flank angle was higher on the other ring. If an average wear of the cutter across the tunnel face of 25mm is considered, the tip width was about 10mm thicker on the other ring than Robbins’. This might not seem dramatic on one ring; however, across the cutterhead this can result in an increase of more than 500mm of steel that you are trying to push through rock. Obviously, this influences the TBM efficiency,” explains Log.
The XHD rings were initially developed for Norway’s Røssåga Hydroelectric Project in 2014, where a 7.23m-diameter Robbins Main Beam TBM encountered massive granitic rock with strengths ranging from 138MPa up to 300MPa and quartz content ranging from 43% to 92%.
The extreme rock properties at Røssåga were causing low cutter life (approximately 100 to 150m3 excavated per cutter), which affected the production of the project. To improve the situation, the cutter department did a detailed analysis of the geology, machine performance, cutter wear and cutter failure mechanisms. Engineers worked together with the steel supplier and heat treatment shop to optimise the cutter ring properties to the geology encountered. After initial trials with several different materials/heat treatments that performed well, one of the versions, XHD, stood out and showed a very promising reduction of the destructive wear of the rings. The rings were installed and were shown to improve performance in extremely hard rock by a minimum of 25%.
Cutter life in 200MPa geology and above was improved by more than 20% compared to standard HD discs. The XHD discs are being tested further at another project that has encountered very hard rock—Vietnam’s Thuong Kon Tum Hydroelectric Power Project (HEPP), and results there are forthcoming.
THE MACHINE MATTERS—AND SO DO THE PEOPLE
Harder rock requires equipment that can stand up to high stresses. “Penetration rate and thrust are exponential functions. The first rule is: Push as hard as you can. The more thrust the better,” says Log.
“From an operational perspective,” says Log, “Boring in rock of 180MPa and above is much different than excavating 100MPa rock. You will see heavy vibration, it will be noisy, and you’ll have low indentation of the excavation face.”
He added that rock of different strengths in the excavation face, including joints or veins, are likely to occur naturally in any rock mass; however, the consequences can be higher in very hard rock conditions. Such conditions require skillful operation of the machine, optimal cutters and a good TBM design. “You need to have a robust machine that supports the cutterhead and the impact of thrust into the rock face. Our machines and cutters are designed with the experience from a significant amount of tunnelling in very hard rock and this has led to robust design that can handle the heavy loads and impact vibrations you can see in extremely hard rock.”
Log added that other TBM design elements can stand a hard rock operation in good stead. Adding protection plates to the cutterhead can protect cutters from damage by blocky rock, while optimal cutter spacing with large diameter (19-inch or 20-inch) cutters in the cutterhead will ensure better penetration rates and longer cutter life. Abrasivity of the rock must also be considered and abrasion plating, such as hardox plating, can be applied to the cutterhead to limit wear.
Log cautions that even the best equipment can run into issues without experienced operators. “The crew operating the TBM is key. They can guide the machine through blocky ground, extremely hard rock, mixed face conditions, and any other challenge that might be encountered.”
MALAYSIA’S MOUNTAINS OF GRANITE
At Malaysia’s Pahang Selangor Water Tunnel, the teams did just that. What is now the longest tunnel in Southeast Asia, at 44.6km, required excavation using three 5.2m-diameter Main Beam TBMs mounted with 19-inch disc cutters. The machines operated in abrasive granitic rock exceeding 200MPa UCS, up to 1,200m below the Titiwangsa Mountain Range. The TBMs were supplied to the SNUI JV – a consortium of Shimizu Corporation and Nishimatsu Construction of Japan, along with local companies IJM Corp and UEM Builders.
Andy Birch oversaw operation of the TBMs during the project, and gave his observations based on the granitic rock he encountered there and at other projects. “The key is to get the best penetration rates. The TBM Operator should be looking for the best advance at the lowest RPM.” He added that the reason for lower RPM is to reduce wear on the outer cutters and periphery of the cutterhead. Tests at multiple sites over several years have shown that a lower RPM achieves the same and often better penetration rates than a higher RPM in hard rock, and substantially reduces wear. Birch gives an example: “Say you have an 8m-diameter cutterhead. The circumference would be 25.12m. At 10 RPM during TBM operation, this would mean 251.2m of travel in one minute. If you can reduce the speed to 8 RPM, the distance would be 200.9 m of rotation – a full 50m less in one minute. That is 3,000m less per hour.”
Birch adds a word of caution about the effects of high RPM in very hard rock: “Some operators just ramp up the head speed to its max and think they are doing well. This point needs consideration: there needs to be a balance between head speed and thrust force. An experienced TBM operator will be able to identify when ground conditions change and react accordingly.”
In Malaysia the TBMs did well despite blocky rock, water inflows, over-break, and fault zones. Advance rates reached up to 49m in one day, 198m in one week, and 657m in one month. For Birch, working with the geology is key: “Impact loading is inevitable when there is a sudden change in ground conditions. If the head is well maintained, in homogenous hard rock our cutters are very robust. When ground conditions suddenly change– maybe there are cavities or one section of the face is softer–the frequency of cutterhead inspections must be reviewed and possibly increased.” He added that in blocky ground, crews should ensure side supports, roof supports or stabilisers are fully operational and in use, as these components keep the cutterhead stable, thereby reducing wear on the machine and cutters.
Above all, Birch stresses the importance of maintenance and inspections when tunnelling in extremely hard rock conditions. In addition to maintenance like monitoring oil levels and taking oil samples in critical equipment like the main bearing, cutter maintenance should not be neglected.
“Having a dedicated cutter crew, with allotted time for cutter change and maintenance, is vital. A trained cutter workshop team can properly rebuild cutters and track them through ID tags to determine trends over time. When trends become clear it’s possible to predict which cutters will need to be changed after so many excavated meters. This increases efficiency and planning during the maintenance shift.” Birch also advocates for cutter rotation from outer areas on the cutterhead to inner positions. “This drastically reduces the number of cutters used. In Malaysia, the contractor wanted to ensure that all cutters were worn to their recommended limit of 35mm before being replaced. So we systematically changed the gauge cutters when they wore down to 10mm and moved them to the inner face areas of the cutterhead where they could wear to the full 35mm without the risk of damage due to the high stresses that gauge cutters face.”
Birch adds that adhering to manufacturer-provided cutter change guidelines is key to the process. The guidelines are there for a reason, and he gives an example: “You cannot install a new cutter adjacent to an old one that is worn to 25mm. This causes premature failure and increased downtime. The guidelines give information on the need to install adjacent cutters with the correct amount of wear between them.” Lastly, he stresses that skilled TBM operators are a necessity in extremely hard rock conditions. “Each cutter has a maximum load value. For 19-inch cutters like those on the Malaysia TBMs the value is 310kN per cutter. Multiply that by the number of cutters on the cutterhead and you get the maximum design thrust force. The thrust force should be properly limited below this number either through the PLC or hydraulically to prevent over-thrusting.” Over-thrusting can damage the cutters and the machine, and steering itself is another factor.
“There should be no excessive steering adjustments. Keep cutterhead movements to a maximum of 3 to 5mm.” Large adjustments can again damage the machine and cutters. With Birch’s experience and guidance, the TBMs at the Pahang Selangor project made successful breakthroughs in 2013 and 2014.
INDIAN ROCK
India’s granitic rock can take a toll on TBMs, particularly over long distances. At the Alimineti Madhava Reddy (AMR) Water Tunnels, two 10m diameter Double Shield TBMs are boring what will be the world’s longest tunnel without intermediate access – 43.5km – when complete. The TBMs, for contractor Jaiprakash Associates (JAL), have encountered zones of quartzite up to 450MPa UCS, layered with shale and granite up to 190MPa.
Robbins Projects Manager for India, Jim Clark, oversaw much of the TBM operation in the difficult conditions at AMR and other projects. He gave his observations: “The two key parameters in hard rock boring are the thrust pressure acting on the cutters and the cutterhead rotation speed. The TBMs being utilised on the AMR project are equipped with 67 cutters with a maximum load of 311kN per cutter, giving a maximum cutter load of 20,837kN. Contractors often mistake this calculation as the maximum allowable operational thrust; however, the weight of the machine and a coefficient of friction must also be applied to the calculation for maximum operating thrust. For the 10m Diameter Double Shield machines used on the AMR project, this equates to an additional 3,700kN; hence, a total of approximately 24,500kN is the maximum allowable thrust pressure. In my experience operating close to maximum thrust – but not over – in hard rock gives the best results regarding advance rates.”
Like Birch, Clark adds that more attention needs to be paid to the cutterhead rotational speed. “In hard rock, increasing the cutterhead RPM up to a certain point will result in increased advance rates due to the simple function of penetration rate per revolution multiplied by number of revolutions per minute. The maximum cutterhead speed of the AMR machines is 7.5 RPM, but we have found that the optimal cutterhead speed is approximately 6.0 RPM in hard rock which gives a penetration rate of approximately 4.2mm/revolution or 1.5 mph.
Any further increase has not generally resulted in improved advance rates, but has resulted in increased cutter wear, which subsequently had an adverse effect on overall production due to increased downtime for cutter changes.”
Clark added that if varying rock strengths are present in the excavation face, the most effective way to prevent impact loading is to reduce cutterhead speed and penetration rate per revolution. While this might seem counter-intuitive, he explains, the time saved on cutter changes more than makes up for the reduction in advance rates.
Maintenance is another key point for Clark, and especially important for both long tunnels and extremely hard rock conditions. “At the start of a project it is preferable to have a set maintenance period each day, say four hours out of each 24-hour period, which enables the crews to become familiar with the maintenance regime. “This equates to approximately 24 hours of maintenance in a 6-day working week. Cutter change time can take up substantially more than four hours per day in hard rock tunnels so once the crews are familiar with the maintenance tasks they can be carried out concurrently with cutter change operations.” He adds that cutterhead inspections should be carried out on a regular basis to enable worn, damaged or blocked cutters to be replaced as soon as possible.
“It may be tempting to extend the period between cutterhead inspections, but boring with one or two blocked cutters can result in a cascade type wipe-out of adjacent cutters. In hard rock this can also result in damage to the cutterhead over the course of just a couple of strokes of boring. The same applies to inspection of the bucket lips.” The TBMs at AMR are currently 86% and 59% complete with their sections of tunnel, and the project serves as an example of the most extreme conditions encountered by TBMs and their crews. “We are a very close-knit team,” said Brigadier Boperai, JAL Joint President, during the TBM operation. “Robbins is very good and professional, and whenever we have needed help they came and assisted us with advice.”
ENSURING SUCCESS
Tunnelling in extremely hard rock is never easy, and Robbins’ current operation of a TBM at Vietnam’s Thuong Kon Tum Hydroelectric Power Project (HEPP) drives that point home. The 17.4km-long tunnel will be the country’s longest once complete, drawing water from the Dak Nghe River to supply electricity to the Central Vietnam region via a 350MW capacity power station.
A 4.5m-diameter Main Beam TBM is boring a section of the tunnel, and have encountered massive granitic rock averaging 270 to 290MPa UCS, exceeding 300 MPa in some areas. As of August 2018, the TBM is more than 85 percent complete with its bore, and is proceeding using the new XHD discs.
“When the quality of rock mass becomes massive and rock strengths ranging from 200 to 300 MPa are encountered during boring operations, parameters like cutterhead RPM and thrust force must be changed to achieve effective advance rate,” said PN Madhan, Robbins engineering geologist for Field Service at the Thuong Kon Tum site. “When varying rock hardness and rock mass characteristics are present, cutterhead RPM should be lowered to avoid impact loading or blocking of cutters. Effective cutterhead openings such as grill bars can also limit the size of incoming rock fragments to prevent over-mucking and damage to conveyors. Hence, grill bars should be monitored every day during cutterhead inspection and necessary maintenance undertaken.”
He added that bucket lips should also be checked during every cutterhead inspection and cleaned of any fines produced during boring operations. “Worn out bucket lips lead to the presence of fines in the cutterhead which in turn wear out the cutterhead structure.”
“Maintenance is the key,” said Greg Adams, Robbins Field Service superintendent at the site. “Regular checking of the lube flow meters and flow rates through the switches is crucial. Keeping the lube oil clean and free from impurities is essential. Regular checking and changing of the filters is also a must, especially the magnetic strainer. This catches any metal that might be in the cavity and is a good indication of bearing wear and condition.”
Adams added that pre-planning is an essential part of their operation, which requires a detailed understanding of the quartz content in the rock, compressive strength, and other factors. “It is always better to consider the rock strength to be higher in nature than the available data from initial investigations.”
Once TBM operation begins, the geology must continue to be monitored. Site geologists fulfill this role by doing regular mapping of the excavation face. Overall, ensuring success in some of the most difficult rock in the world requires pre-planning with proper machine design, good cooperation by all parties involved, a knowledgeable crew, and dedicated maintenance.
More than that, says Log, it also takes some experimentation on the part of the contractor or those overseeing the TBM operation.
“So much is dependent on the rock at a specific project. Contractors should experiment with the parameters of TBM operation, do regular penetration tests, vary the RPM, vary the thrust. See what gets the best cutter life and the best TBM performance.
“Take the time to do the testing to see what works.”
